Oracle Blog

Virtualization and Cloud Computing

Wednesday Nov 19, 2008

This is in response to a comment on the previous post with regards to the benchmark demonstrating no penalty hit for using a NAS storage device like AmberRoad instead of a LocalFS for a Web 2.0 application. The comment stated:

"I respectfully disagree with your comment that there is no performance penalty
with NFS...You show a 12% increase in Processing utilization which is an
overall performance hit. You are doing approx. the same amount of work
but chewing up more CPU...while your users are still the same, if scaled to 100%
Util, NFS wouldn't allow as many users as LocalFS, because you are turning Blocks
into packets into blocks, which is SLOW."

As per the subject matter experts at Sun, this is a fairly common argument in regards to idle %. We don't know if
it's wrong in this case or not - but idle is not always an indicator of
future performance or perceived headroom, it is wrong to assume so. The
reality is you don't know how much more you can get from your system as
configured unless you push it to do so. % idle is a wrong metric to focus on (very common). The metric that matters in this case
is supporting concurrent users with response times for all transactions
falling within the guidelines. The NAS solution does this just fine for
less $$. Also as per detailed data here, the io latency reported by iostat is
the same for both DAS and NAS - therefore the statement that NFS is slow
is not backed up by evidence.

To the question if NFS io ultimately cost more? Sure - but the missing point here is
that it doesn't matter with this test. Might it hurt us down the road
pushing to 100%? Maybe - but with a cheaper solution, and load scaled
to 75% of the DAS solution, user response time was fine (this is what
matters). And to use the analogy of idle, with over 25% cpu free, we believe it could scale another 800 users, equaling the DAS result. Once we arrange for hardware (better network and drivers) to push the load beyond 2400 users, we would know for sure. Pl. stay tuned.

Lastly, do most customer environments really run at 100%? Will the
purported "NAS penalty" really affect them? This shows that even if
they run at 75% (pretty high for most environments I've seen), it's not
an issue. HTH.

Its also evident from the benchmark data that you don't suffer a performance penalty for using
NAS. There is a fairly common impression that performance could/would
be slower than DAS, this shows that it's just not true with this
environment.